Most horizontal wells in unconventional reservoirs are drilled in the direction of the minimum stress. The preferred far-field fracture orientation thus favors hydraulic fractures transverse to the wellbore. The near-wellbore stress concentration, however, sometimes favors the initiation of fractures in a plane defined by the well axis. Transverse and axial hydraulic fractures can thus both initiate in some situations and can cause significant near-wellbore tortuosity. The presence of both transverse and axial fractures in the near-wellbore region increases the tortuosity of the flow path within the created fractures and thus, for example, significantly perturb proppant placement.
Most wells in unconventional shale reservoirs are preferably drilled horizontally in the direction of the minimum horizontal stress in order to obtain multiple transverse hydraulic fractures after well stimulation. The cylindrical nature of all wells induces elastic stress concentrations with radial and tangential components that are dependent on borehole fluid pressure in contrast to the axial stress component that is independent of it. Thus, the increase of borehole pressure will eventually generate tensile tangential stresses that may overcome tensile strength and initiate longitudinal fractures (also referred to as axial fractures herein) in a plane defined by the well axis. In contrast, the initiation of a transverse fracture requires the generation of axial tensile stresses from either thermoelastic perturbations, or the pressurization of preexisting natural defects (i.e. cracks), perforations, notches or plug seats. In practice, both transverse and axial hydraulic fractures can initiate from horizontal wells as reported by field observations for both open, cased holes as well as laboratory experiments. When initiated, axial fractures can either reorient themselves to become orthogonal to the minimum stress if they continue to propagate or stop their propagation, depending upon their competition with transverse fractures. The presence of axial or both axial and transverse fractures can lead to higher treating pressures, challenges for proppant placement and increased potential for screenouts. Minimizing axial fractures is therefore of interest for horizontal well stimulation applications.
This problem has been studied using laboratory experiments on hydraulically fractured rock blocks and numerical simulations of fracture initiation pressures based on either a linear elastic strength criteria or a linear elastic fracture mechanics criteria. Each mode of propagation has been studied independently, but the coupled solid-fluid modeling of hydraulic fracture initiation and propagation from a borehole comprising axial and transverse fractures has not been documented.
The most striking field observation of the presence of both axial and transverse fractures in an open horizontal well can be shown on an image log from the Barnett field. FIG. 1 is an image log of a Barnett horizontal well drilled in the direction of the minimum horizontal stress showing fractures in both longitudinal and transverse directions (dark gray). The two longitudinal fractures run along the wellbore at 180 degrees from each other at the top and bottom of the borehole. They are intersected by a series of evenly spaced, small transverse fractures of similar lengths. The background shows shale beddings (lighter gray) as parallel to the wellbore. The horizontal well is drilled in the direction of the minimum principal stress in a field that is known to have a low horizontal stress differential. While the axial fractures have been interpreted as classical drilling-induced fractures from drilling mud pressure variations, the transverse fractures have been interpreted as thermally-induced fractures from the cooling effect due to temperature difference between the drilling mud and the formation. This example highlights the fact that in a low horizontal stress differential environment, small stress perturbations can create axial and transverse fractures originating from the open hole that can serve as seed cracks for future hydraulic fractures. One important missing parameter from such image log observation for hydraulic fracturing considerations is the depth of such fractures away from the borehole wall.
Historically, researchers observed the effect of horizontal stress anisotropy with laboratory experiments using open horizontal wells in cement blocks under polyaxial stress, where low horizontal stress differential mostly led to both transverse and axial fractures as shown in FIG. 2, while high horizontal stress differential mostly favors transverse fractures. The previous observations were moderated when studying the impact of the product of the injection rate and fluid viscosity—at higher injection rates and viscosities, the fractures showed the tendency to initiate along the wellbore, irrespective of the horizontal stress differential. FIG. 2 is a schematic diagram of longitudinal and transverse fractures from a horizontal borehole in low stress anisotropy case. U.S. Pat. No. 7,828,063 provides some additional details and is incorporated by reference herein.
For a cased horizontal wellbore with perforations, it has long been recognized that fractures can initiate as a “starter fracture” at the base of the perforations, then to develop into a “primary” longitudinal fracture of limited length against the intermediate stress, and finally become a “secondary” transverse fracture that initiates at right angle to the longitudinal fracture (FIG. 2). Situations where the borehole is inclined with respect to the principal stresses have also been investigated and lead to the two types of fractures with additional fracture complexities. Experimental studies have also shown that the creation of axial fractures from perforations can be minimized if the perforation interval is less than four times the diameter. Alternative to line or spiral perforations, transverse notches can also be created by jetting tools in order to favor transverse fractures. Notches (also known as cavities) may be created using a perforation device such as the ABRASIJET™ device which is commercially available from the Schlumberger Technology Corporation of Sugar Land, Tex. A perforation device may include an operational device, a perforation tunnel tool, a shaped charge tool, a laser based tool, a radial notching tool, a jetting tool, or a combination thereof. Details for forming a notch (i.e. removing a region of a formation) and using the device are provided in U.S. Pat. No. 7,497,259, which is incorporated by reference herein. Additional details are provided by United States Patent Application Publication Number 2013-0002255 and U.S. patent application Ser. No. 13/402,748. Both of these applications are incorporated by reference herein. Multiple perforations are described in U.S. Provisional Patent Application Ser. No. 61/863,463 which is incorporated by reference herein.
FIG. 3 is a schematic diagram of fractures initiated from perforated cased horizontal borehole and is redrawn from photo of laboratory test on cement blocks under polyaxial stress. This typical fracturing process starts at the base of the perforations, then continues with primary axial fractures and secondary transverse fractures.
Most analysis related to the type of fracture obtained for a particular well orientation and stress field are based on the computation of the stress perturbation around the well and the use of a stress-based tensile failure criteria tailored for defect free open holes, for the effect of perforation tunnels, and for the effect of material anisotropy. Such an approach provides an order of magnitude for the fracture initiation pressure and the most likely type of fractures to be expected (axial or transverse). However, if one or both type of fractures are favored at the borehole wall due to the stress concentration, such a stress analysis does not reveal anything about their extent in the formation. More specifically, depending on the situation, although longitudinal fractures may initiate first, higher energy may be required to propagate them further in the formation compared to transverse fractures. Ways to more effectively estimate and implement fracturing regimes including notch introduction and fluid introduction are needed.